System and method for spectral node splitting in a hybrid fiber optic-coaxial cable network

ABSTRACT

A system and method for spectral node splitting in cable TV network based on hybrid fiber-coax architecture is disclosed. Two-way optical signals carrying video and/or multimedia content are received at a signal distribution node. Portions of the received signals are processed according to the functionality the direction thereof. Certain frequency ranges are passed intact while specific different downstream and specific different upstream frequency bands are converted in a manner as to be allowed to be multiplexed into an extended broadband signal. The frequency bands within the extended broadband signal are distributed selectively among separate branches of the network. The frequency bands dedicated to the upstream traffic are received selectively from the separate network branches to be back converted and transmitted upstream to the network controller.

RELATED APPLICATIONS

The present application is generally related to co-pending PCTapplication No. PCT/IL00/00655 by Zeev Averbuch and Dr. Hillel Weinsteinentitled “System and Method for Expanding the Operational Bandwidth of aCommunication System”, filed 16 Nov. 2000 which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to hybrid fiber optic-coaxialcable TV networks, and more particularly, to the application of aspectral node splitting technique designed to reduce the number of cableTV subscribers sharing the same transmission frequency spectrum.

2. Discussion of the Related Art

The capability to provide cable TV subscribers with a greater number ofrevenue-generating electronic services has become increasingly importantfor commercial success in the entertainment as well as thetelecommunications industries. The long distance telecommunicationexchange companies, the local exchange companies, and local cable accesstelevision (CATV) companies, the satellite communications companies areall seeking the right combination of technologies to provide additionalservices to the their subscribers. The additional services potentiallyinclude video-on-demand, pay-per-view, interactive television and games,videoconferencing, video telephony, CATV, Internet access, onlinecommerce, and telephone services. In order to provide any combination ofthe above-mentioned services in an economically viable manner, adistribution network of substantial capacity is required. Capacity, inthis sense, refers primarily to the information carrying capability,which is substantially related to the transmission frequency spectrumbandwidth of the transmission medium.

A transmission medium having the capacity needed to provide the requiredservices is optical fiber. Although it is expected that in some futurepoint in time certain users may have optical fiber running into theirhomes or offices, under the present circumstances it is not economicallyfeasible to deploy an all fiber network infrastructure at one time.Therefore, alternative network architectures are being conceived,considered, and selectively implemented continuously. Certain proposedarchitecture types indicate a general trend toward the deployment offiber backbone architecture.

A network architecture, which is presently considered sufficientlyeconomical and consequently is being implemented widely is a hybridfiber optic-coaxial cable (HFC) network. In an HFC network feeder fibersrun from a head-end to an HFC distribution node remotely located inrespect to the head-end. At the HFC distribution node, the fiber linesare interfaced with a coaxial cable distribution network thatdistributes the signals transmitted across the feeder fibers throughcoaxial cables to a plurality of subscribers.

FIG. 1A illustrates in a simplified manner a full coaxial networkarchitecture. Analog or digital broadband signals are transmittedbetween a head-end 10 and a multiplicity of coaxial distribution nodes12, 14, 16, 18, 20, 22, 24, 26 through a coaxial backbone. The coaxialbackbone includes various cascaded RF components (not shown) such asline driver amplifiers for example, which are used to boost the signalstrength in order to compensate for the attenuation of the signals. Atthe coax distribution nodes 18, 20, 22, 24, 26, the broadband signal issuitably split off to separate coaxial branches in order to be fed intoappropriate coaxial feeder lines to be further distributed to theindividual subscribers. In order to increase the number of subscribersserved by each coaxial distribution node, coaxial feeder linesassociated with the coaxial distribution nodes 18, 20, 22, 24 includemultiple RF components 28, 30, 32, 34, 36, 38, 40, 42, 44, and 46.

In the currently operating HFC networks all the coaxial backbones werereplaced by fiber optic transmission plants. A fiber plant includesfiber-optic lines, and specific opto-electronic components such asoptical transceivers, optical amplifiers, optical switches and the like.The fiber optic plant of the network terminates at specific hybridfiber-coax distribution nodes, which are coupled to the separate coaxialbranches. The branches include coaxial feeder lines and associated RFcomponents. The feeder lines are operative in carrying the broadbandsignal to the subscribers that are connected to the feeder line viaspecific connection points referred to typically as taps.

The optical signal carries encoded information units. The signal istransmitted through the fiber plant in the form of light signals atspecific wavelengths. At the HFC distribution nodes the optical signalsare converted into RF signals and are forwarded via the coaxial branchesto the subscribers. With reference to FIG. 1B a highly simplifiednetwork of this type of architecture is illustrated. Optical signals aretransmitted from a head-end 50 via an optical transceiver 52, a fiber54, a hub 56, and a fiber 58 to a hybrid fiber-coax distribution node60. The node 60 includes an optical transceiver and suitable converterunits to convert the optical signals into RF signals. The RF signals aretransmitted to the subscribers through appropriate coaxial branches,which include coax feeder lines, coax distribution nodes 62, 64, 66, 68and suitable RF components 70, 72, 74, 76, 78, 80, 82, 84, 86, 88.

The HFC architecture has a number of drawbacks. One disadvantageconcerns the relation of the allocated transmission frequency bandwidthfor the delivery of the downstream traffic to that of the upstreamtraffic. As a result of the limited bandwidth of the coaxial plant cableHFC architectures are inherently asymmetrical, with as much as 95percent of total capacity dedicated to the downstream (head-end tocustomer) traffic and less than 5 percent is available for upstream(customer to head-end use). In addition, the downstream traffic is apoint-to-multipoint architecture while the upstream traffic ismultipoint-to-point. As the new electronic services in the markettypically provide more interactive capabilities to the subscribers,networks that implement those services require a substantially widerupstream bandwidth to carry the increasingly heavy upstream traffic.

Another disadvantage of the HFC architecture relates to the differencesin the transmission capacities of the fiber plant and the coaxial plantof the network. The transmission capacity of the coaxial section issubstantially lower than the transmission capacity of the fiber opticsection. Thus, the overall transmission capability of the HFC is limitedby the transmission characteristics of the coaxial section. This createsa “bottleneck” where the high-capacity fibers are coupled to thesubstantially lower capacity coaxial cables. As a result of the reducedcapacity the coaxial section is capable of serving relatively fewsubscribers, and thus, requires more feeder fibers to terminate at thehybrid fiber-coax distribution node.

The operators of the presently active HFC systems make every effort tointroduce operational and technological improvements into theirnetworks' architecture in order to eliminate, to alleviate, or to reducethe negative effects caused by the above-described drawbacks. A growinglist of techniques, both operational and technological, are or will beavailable to make the flow of traffic within HFC networks more efficientin both directions. One such a group of techniques concerns spectrallyefficient modulation methods, referred to typically as “higher ordermodulations”. For example, replacing the currently prevalent QPSKmodulation of the upstream traffic with the spectrally more efficient16-QAM modulation method may substantially double the total throughputin the upstream direction. Other similar advanced upstream modulationsthat being proposed or developed are the 256-QAM, the 128-QAM, the64-QAM, the Advanced PHY, the S-CDMA, and the F-TDMA, and the like.

A completely different technique, which is being accepted for use in theHFC networks is referred to as Physical Node Splitting (PNS). PNStypically halves or quarters the number of homes and businesses sharingthe transmission frequency spectrum or certain portions thereof. Mostcurrent opto-electric node equipment comes with four output legs whereeach leg connects to a coaxial section of a network. That yields twopotential splits such as from about 500 subscribers to about 250subscribers, then from about 250 subscribers to about 125 subscribers.Typically, in order to split a node physically, the re-location of theHFC distribution nodes, the addition of HFC nodes, the laying ofadditional fiber and/or coaxial cables and the installation ofadditional equipment such as lasers, transmitters, receivers, and thelike, are required. The PNS method will be described next in associationwith FIG. 2.

With reference to FIG. 2 an optical signal is transmitted from thehead-end 90 via a fiber 92 to a hub 94. The hub 94 is coupled via thefibers 96, 98, 100, 102 to the HFC distribution nodes 104, 106, 108, 110respectively. The HFC nodes 104, 108, 110 are coupled to separatebranches (not shown) of the coaxial plant. The node 106 is physicallysplit by the installation of additional opto-electric and RF components.The node 106 receives the optical signal, converts the signal to an RFsignal and distributes the signal to four respective coaxial branches109, 111, 107, and 105. The branches 109, 111, 107, and 105 are coaxialcables, which distribute the RF signal to the subscribers associatedwith the branches. The branch 109 utilizes cascaded RF components 112,114, 116, 118 to drive the signal to the subscribers. Similarly, thebranch 111 utilizes cascaded RF components 128, 130, 132, 134, and 136to drive the signal to the subscribers, the branch 107 utilizes cascadedRF components 120, 122, 124, 126 to drive the signal to the subscribers,and the branch 105 utilizes cascaded RF components 138, 140, 142, 144,and 146 to drive the signal to the subscribers.

Typically, the location of the operational HFC distribution node doesnot always identical with the location of the “edge” of the fiber-opticsection of the network. Thus, the activities of the Physical NodeSplitting usually involve of the laying of additional fiber. The layingof fiber lines demands extensive excavation for the laying of conduitpipes to hold the fiber lines, in addition to the laying of theadditional coax lines. In situations where the HFC distribution mode isplaced after the “last mile” amplifiers, a change of the amplifierdirections as well the change of the direction of passive elements isrequired. FIG. 3 demonstrates the extension of the fiber plant inassociation with the physical node splitting method. The HFCdistribution node 168 is the end of the original fiber plant that feedsthe coaxial branch 150. In addition, the node 168 feeds the opticalsignal through a fiber 151 to the HFC node 170. The optical signal istransmitted through the fiber 172 to the HFC 174. The HFC 174 is coupledto the coaxial branch 152 via a splitter 176. The HFC 174 converts theoptical signal to an RF signal and transmits the signal downstream tothe subscribers via the RF components 178, 180, 182, and 184. Upstreamsignals originated by the subscribers are transmitted upstream via theRF components 198, 196, and 194 having reverse directionality.

Currently, in order to achieve effective physical node splitting theoperators are obliged to utilize labor-extensive methods. Typically thefiber nodes should be moved geographically closer to the subscribers'premises, and the coaxial branch should be physically split toadditional branches. The process involves complex physical operationssuch as excavation for the deployment of underground pipes for theplacement of the new fiber and the new coax, the re-location or theaddition of HFC nodes, and the consequent re-organization of the coaxialbranches. Thus, node splitting is a highly complicated process thatrequires careful planning and organization of the work to be done. Asthe deployment typically performed in metropolitan areas, it is alsohighly desirable that the process be completed within a predeterminedprecise time frame. Consequently the operation involves considerableexpenses. Current estimates for physically splitting a single node arein the order of tens of thousands of US dollars. Thus, a keyconsideration for network design regarding the desirability of physicalnode splitting is to be able, as much as possible, to match theequipment deployment expense with the expected revenue from the service.

The general objective of the physical node splitting method is toincrease the overall transmission capacity of the network. It would beeasily perceived to one with ordinary skill in the art that a clear andpresent need exists for an improved, non-labor-extensive, andcost-effective system and method that could be implemented instead ofthe physical node splitting method in a hybrid fiber-coax cable network,such as to achieve the same general objective.

SUMMARY OF THE PRESENT INVENTION

One aspect of the present invention regards a communications networkaccommodating at least two subscribers linked via at least onecommunications network branch to at least one content distribution unitwhere the content distribution unit is feeding a two-way radio frequencysignal to the at least one communications network branch, and a systemof spectrally splitting the at least one distribution unit in order toreplace the at least one communications network branch by at least twoseparate communications network branches. The system contains theelements of: a) at least one extended converter unit to receive via anat least one optical signal conduit an at least one two-way opticalsignal carrying content information having a pre-determined transmissionfrequency bandwidth, to form a RF signal having a substantially extendedtransmission spectrum bandwidth, to convert pre-determined downstreamportions of the at least one two-way optical signal into at least twodifferent pre-determined RF transmission frequency bands, and tointroduce the at least two converted different RF bands into amultiplexed downstream RF signal having a substantially extendedtransmission spectrum bandwidth, b) at least two extended amplifiers toselectively pass at least two pre-determined RF bands from themultiplexed RF signal having a substantially extended transmissionspectrum bandwidth downstream to the at least two differentcommunications network branches, and c) at least one back converter unitto receive via the at least one communications network branch at leastone branch specific RF signal, to extract from the at least onebranch-specific RF at least one predetermined upstream frequency band,to convert the at least one branch-specific upstream RF band to apredetermined upstream frequency band, and to introduce the convertedupstream RF band into the multiplexed RF signal having a substantiallyextended transmission bandwidth to be delivered upstream to the at leastone content distribution unit.

A second aspect of the present invention regards a communication networkaccommodating at least two subscribers linked via at least onecommunications network branch to at least one content distribution unit,the content distribution unit is feeding a two-way RF signal to the atleast one communications network branch, a method of spectrallysplitting the at least one content distribution unit in order to replacethe at least one communications network branch by at least two separatecommunications network branches. The method comprising the steps of: a)receiving at least two optical signals carrying encoded contentinformation in predefined different downstream frequency band, b)converting the at least one pre-defined downstream frequency band to apre-determined converted frequency band within the combined broadbandsignal, c) converting the at least one pre-defined upstream frequencyband to a pre-determined frequency band within the combined broadbandsignal, d) multiplexing the CATV signal and the at least two convertedfrequency band into a combined broadband signal having a substantiallyexpended frequency range, e) selectively distributing the converteddownstream frequency band in the combined broadband signal to separatecommunications network branches, f) selectively receiving at least twoupstream frequency bands included in the combined broadband signal fromseparate communications network branches, and g) converting the at leasttwo upstream frequency bands to pre-defined frequency bands.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1A is a schematic illustration of coaxial cable plant, as known inthe art; and

FIG. 1B is a schematic illustration of a hybrid fiber/coaxial cableplant, as known in the art; and

FIG. 2 is a schematic illustration of a hybrid fiber/coax cable nodesubsequent to the performance of physical node splitting (PNS), as knownin the art; and

FIGS. 3 demonstrates the new fiber location and amplifier reversalrequired for the physical node splitting (PNS), as known in the art; and

FIG. 4A illustrates the existing CATV transmission frequency spectrum,as known in the art; and

FIG. 4B illustrates the frequency plan of the downstream frequencyspectrum band utilized by the spectral node splitting, in accordancewith the preferred embodiment of the present invention; and

FIG. 5 illustrates the frequency plan of the upstream frequency spectrumband utilized by the spectral node splitting, in accordance with thepreferred embodiment of the present invention; and

FIG. 6 illustrates the frequency bands utilized by the spectral nodesplitting, with interleaved upstream and downstream bands, in accordancewith the preferred embodiment of the present invention; and

FIG. 7 is a schematic block diagram of a hybrid fiber/coax distributionnode and associated coaxial branches subsequent to the performance ofthe spectral node splitting, in accordance with the preferred embodimentof the present invention; and

FIG. 8 is a schematic block diagram of the extended amplifier/converter,in accordance with the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

PCT Patent application Serial No. PCT/IL00/00655 by Zeev Averbuch andDr. Hillel Weinstein entitled “System and Method for Expanding theOperational Bandwidth of a Communication System”, within which a methodand system for the substantial expansion of the usable bandwidth of aCATV network is disclosed, is incorporated herein by reference.

A system and method of spectral node splitting in a hybrid fiberoptic/coaxial (HFC) network is disclosed. Although in the preferredembodiment of the present invention the spectral node splitting isimplemented in a cable television communications network (CATV)primarily distributing video and/or multimedia content it would beeasily understood that in other preferred embodiments the proposedsystem and method could be applied to diverse other communicationsnetworks such as satellite communications networks, Local Area Networks(LANs), Wide Area Networks (WANs), or any other communications networkinvolving the two-way delivery of information units between a centrallocation and users located remotely in respect to the central location.The system and method proposed could also be utilized in a peer-to-peernetwork operative in the delivery of information units between remote orlocal users. The content delivered by the relevant communicationsnetworks could be of any distributable material, such as voice, images,video, data structures, graphics, executable code, text, astronomicaldata and the like, The following description of the preferred embodimentis meant to provide a ready understanding of the present invention. Theembodiment-specific descriptions of the various components andoperational methods are not meant to be limiting. The limits of thepresent invention will be defined only by the following claims.

Typically an HFC network includes a fiber optic plant and a coaxialplant coupled via specific interface units. The fiber optic plant isutilized as the backbone medium of the network and it is operative inthe two-way delivery of information units, encoded into optical signalsand modulated at specific light wavelengths, across fiber optic cablesto distribution nodes located on the edge of the fiber optic plant. Thefiber plant includes fiber-optic delivery media (optical conduits) andsophisticated opto-electronic equipment such as lasers, transmitters,receivers, amplifiers, repeaters, switches and the like. The opticalelements are utilized to suitably manipulate and maintain the requiredcharacteristics of the optical signal. The coaxial plant is used as thetwo-way delivery and distribution media between the fiber plantdistribution nodes and the subscribers of the network. The coaxial plantis coupled to the fiber-optic plant through specific HFC distributionnodes. The coaxial plant includes coaxial cables as the transmissionmedium, and associated RF components such as amplifiers, switches, andthe like to suitably manipulate and maintain the requiredcharacteristics of the RF signal representing the information unitsdistributed. The HFC distribution node is operative in the conversion ofthe optical signals to a RF signals for the downstream traffic and inthe back-conversion of the RF signals to optical signals for theupstream traffic. The transmission capabilities of the fiber optic plantand the coaxial plant are substantially different. Currently a typicalhybrid fiber optic/coaxial cable plant is operative in the delivery of asignal having a transmission frequency bandwidth of up to about 860 MHZ.Although the capacity of the fiber optic plant is substantially higher,the entire hybrid transmission path is limited by the maximumtransmission capacity of the coaxial plant.

Each HFC distribution node services a plurality of homes. The number ofhomes passed is also referred to as the “node size”. Since the inceptionof fiber optic deployment several years ago node sizes have decreasedsteadily. Downward opto-electronics pricing trends have enabled cableoperators to reduce node sizes from 10,000 homes passed or more to thecurrent norm of 2000 homes per node. This number is driven by a numberof factors including economics, performance and capacity. The additionof service packages implemented by the cable operators in the networkfrequently effects heavier traffic. In order to reduce the load on anoverloaded HFC distribution node a specific process referred to as “nodesplitting” is performed. Node splitting involves the sub-dividing of adistribution node into two or more new distribution nodes with theresulting decrease in the number of homes serviced by each node. Nodesplitting is currently performed by physical methods such as the layingof new optical cables, new coaxial cables, the installation of new HFCdistribution nodes, the re-location of existing HFC distribution nodes,and the addition of opto-electronic equipment such as lasers, opticaltransceivers and the like.

The present invention proposes a novel system and method for spectralnode splitting. Spectral node splitting involves the formation of asupplementary usable radio frequency spectrum range of about 1 to 3 GHzhaving a bandwidth of about 2 GHz. The supplementary frequency range isappended to the regular CATV frequency spectrum range of about 5 to 860MHz having a bandwidth of about 855 MHz. The combined frequency spectrumrange spans the frequency spectrum the about 5 MHz to 3 GHz. Thesupplementary frequency spectrum serves as an overlay to carry the RFsignals within the coaxial plant from the HFC distribution nodes to thesubscribers. The system and method regarding the formation, processing,and maintenance of the supplementary radio frequency spectrum range ofabout 1 to 3 GHz is disclosed in the related co-pending PCT application,which is incorporated herein by reference. The separate signals sent tothe subscribers from the HFC distribution node and received from the HFCdistribution node from the subscribers are multiplexed into an RF signalhaving a transmission bandwidth of about 3 GHz. The signal includes theoriginal CATV frequency band and additional downstream and upstreambands inserted into the supplementary frequency range. The signal isdistributed among the separate coaxial branches such that each branchreceives the original CATV frequency band and its own specificdownstream and upstream frequency band. The distribution of themultiplexed frequency ranges to each branch is performed by thefollowing steps: a) the optical signals carrying the information unitsdelivered from the upstream and downstream fibers are collected, b) theupstream and downstream frequency ranges are up-converted tospecifically predetermined frequency ranges included in the extendedfrequency spectrum bandwidth, c) broadband amplifiers capable ofhandling the extended frequency spectrum between the subscriber and aback-conversion point are installed. d) at a down-conversion unit thebranch-specific frequency ranges carrying upstream information withinthe supplementary frequency range are down-converted in a predeterminedmanner. The about 50 to 660 MHz or the about 50 to 500 MHz range ispassed without being converted such as not to interfere with theregular, commercial CATV content, while the coaxial branches-specificfrequency ranges included in the supplementary frequency bandwidth areconverted back to be inserted into the frequency range of the about 660to 860 MHz band and of the about 5 to 42 MHz band.

The system and method proposed by the present invention enablessplitting of an HFC node without having to resort to extensive diggingfor the laying of new fiber and/or coaxial cables. The system and methodalso allows node splitting without having to install additional HFCdistribution nodes or re-locate existing nodes.

Referring now to FIG. 3 the existing CATV transmission frequencyspectrum spans a frequency range of about 5 to 860 MHz. The frequencyrange includes several sub-bands having predetermined functionality andtransmission directionality. The about 5 to 42.65 MHz sub-band 402 isused for the upstream traffic of typically advanced interactiveservices, such as Internet access, telephony applications, interactivetelevision, Video-on-Demand, and the like. The about 55/85 to 550 MHzfrequency sub-band 404 is utilized typically for analog televisionbroadcasts. The about 550 to 650 MHz sub-band 406 is typically used forthe delivery digital television content and the about 660 to 860 MHzsub-band 408 is used for the downstream traffic of advanced services,such as Internet access, telephony applications, Interactive television,Video-on-Demand, and the like. Due to the substantially high level ofinteractivity required by the services utilizing the 402 and 408 bandsthese bands can provide only a limited capacity per subscriber whereutilized by a plurality of subscribers.

FIG. 4 illustrates the frequency plan operative in the arrangement thedownstream frequency bands within the supplementary frequency band. Thesupplementary frequency range extends from about 1000 MHz to about 2000MHz. In the preferred embodiment of the present invention the about 1250MHz to about 1950 MHz band 232 consists of the four frequency bands 234,236, 238, 240 where the bands are suitably separated by guard bands.Each of the frequency bands 234, 236, 238, 240 is allocated a frequencyrange of about 160 to 200 MHz. The frequency bands 234, 236, 238, and240 are used to carry the original about 660 to 860 MHz downstreamportion of the signal split four ways. The frequency bands are alsodesignated on the drawing as D1 (234) for the first downstream band, D2(236) for the second downstream band, D3 (238) for the third downstreamband, and D4 (240) for the fourth downstream band. In the preferredembodiment of the present invention the four frequency bands 234, 236,238, and 240 include information units having identical informationcontent. In other preferred embodiments of the invention the bands 234,236, 238, 240 could carry branch-specific information units.

FIG. 5 illustrates the frequency plan operative in the arrangement ofthe upstream frequency bands within the supplementary frequency band.The supplementary frequency range extends from about 1000 MHz to about2000 MHz. In the preferred embodiment of the present invention the about2250 MHz to about 29500 MHz band 222′ consists of the four frequencybands 224, 226, 228, 230 where the bands are suitably separated by guardbands. Each of the frequency bands 224, 226, 228, 230 is assigned afrequency range of about 30 MHz. The frequency bands 224, 226, 228, and230 are used to carry the branch-specific upstream signals from theentire set of subscribers of a specific coaxial branch to theback-conversion unit. By definition the upstream frequency bands 224,226, 228, 230 carry different information content. The frequency bandsare also designated on the drawing as U1 (224) for the first upstreamband, U2 (226) for the second upstream band, U3 (228) for the thirdupstream band, and U4 (230) for the fourth upstream band. Although inthe preferred embodiment of the invention only four upstream frequencybands associated with four separate coaxial branches are shown it wouldbe easily understood that in other preferred embodiments any number offrequency bands associated with any number of separate coaxial branchescould be implemented.

The arrangements of the supplementary extended frequency range such asspectrum, bandwidth and center frequencies are flexible. Alternativefrequency plans could be used with interleaved upstream and downstreambands. FIG. 6 illustrates one alternative frequency plan. Thesupplementary frequency range extends from about 1000 MHz to about 2000MHz. In the preferred embodiment of the present invention the about 2250MHz to about 29500 MHz band 222 consists of the eight frequency bands244, 246, 248, 250, 252, 254, 256, 257 where the bands are suitablyseparated by guard bands. The bands 244 (D1), 248 (D2), 252 (D3), and256 (D4) are each assigned a frequency range of about 160 to 200 MHz.The bands 244, 248, 252, and 256 are designed to carry the four-waysplit identical downstream information. The bands 246 (U1), 250 (U2),254 (U3), and 267 (U4) are each assigned a frequency range of about 30MHz. The bands 246, 250, 254, and 257 are designed to carrynon-identical upstream information from four different coaxial branches.

Referring now to FIG. 7 which is a schematic block diagram illustratingthe architecture of the HFC plant following the implementation of theSpectral Node Splitting technique. The optical signal is fed downstreamfrom the head-end 186 through a fiber optic transceiver 188, via a fiber190 to a hub 192. The optical signal is transmitted from the hub 192through a fiber 194 to the HFC distribution node 196, which includes anoptical transceiver and additional components operative in convertingthe optical signal to a radio frequency signal in digital format. Noteshould be taken that although on the drawing discussed only a singlefiber is shown, in a realistic environment more than one fiber could beused. Typically the optical signal is transmitted downstream through onefiber and passed upstream through a second fiber. Different opticalsignals having identical or different frequency bandwidth, differentmodulation, frequency plans, and information content could be carried bya single fiber where the different signals would be carried together asseparate wavelengths of light in a multiplexed signal using densewavelength division multiplexing (DWDM). Alternatively the differentoptical signals could be carried by different fibers. The HFCdistribution node 186 is the “edge” unit of the fiber optic plant. TheHFC distribution node 186 receives the optical signals from the head-end186, converts the optical signals to digital RF signals and feds thesignals downstream to the coaxial distribution units 200, 202, 204, 206,and 208. Between the fiber plant and the coaxial plant a specificallydesigned and developed converter/amplifier unit 198 is placed.Converter/amplifier unit 198 processes the RF signal received from theHFC distribution node 196. The unit 198 is operative in suitablyrelocating the downstream frequency ranges extracted from the convertedRF signal into predefined frequency ranges within the supplementaryfrequency spectrum range. For each specific coaxial band a specifictransmission frequency range is allocated. The RF signal modulated intothe supplementary frequency spectrum range is carried to the RF units200, 202, 204, 206, and 208. The units 200, 202, and 204, arespecifically designed broadband amplifiers operative in handling thesubstantially extended transmission frequency spectrum of about 5 MHz to3 GHz. The units 200, 202, and 204 feed the specific coaxial branchesD4+U4, D3+U3, D2+U2. The RF units 200, 202, and 204, are implementedsuch that each filters out the frequency range not intended for its ownbranch. The suitable filtered RF signal is fed through theback-converter units 210, 220, 228 that are placed at the terminationpoints of the coaxial branches D4+U4, D3+U3, D2+U2, respectively. Thecoaxial branch D4+U4 associated with the broadband amplifier 200 and theback-converter 210 includes a coaxial cable with a set of cascaded CATVamplifiers 212, 214, 216, and 218. The branch D4+U4 delivers thebranch-specific signal to subscribers connected to taps along thecoaxial cable. Similarly, the coaxial branch D3+U3 associated with thebroadband amplifier 202 and the back-converter 220 includes a coaxialcable with a set of CATV amplifiers 220, 222, 224, and 226. The branchD3+U3 delivers the branch-specific signal to subscribers connected totaps along the coaxial cable. The coaxial branch D2+U2 associated withthe broadband amplifier 204 and the back-converter 228 includes acoaxial cable with a set of cascaded CATV amplifiers 228, 230, and 232.The branch D2+U2 delivers the branch-specific signal to subscribersconnected to taps along the coaxial cable. The branch D1+U1 delivers itsown branch specific signal to subscribers connected to tap along thecoaxial cable. The converter/amplifier 198 located at the beginning ofthe coaxial section performs the following operations in respect to thedownstream bands: a) the CATV downstream frequency bands (the about 50to 660 MHz and the about 660 to 860 MHz) are passed intact; b) the threeadditional downstream frequency bands are converted into thepredetermined frequency bands (D4, D3, D2) within the supplementaryfrequency band having a substantially extended bandwidth, c) the entiredownstream band of about 5 MHz to 3 GHz is amplified and fed downstream.The converter/amplifier 198 performs the following operations in regardto the upstream bands: a) the CATV upstream frequency band (the about 5to 42 MHz) is passed intact as the band includes relatively lowfrequencies, which could arrive easily from the farthest branch of thenode D1+U1, b) the three additional upstream frequency bands areconverted back to the CATV spectrum's original upstream bandfrequencies.

The back-converters 210, 220, and 228 placed at the termination of thecoaxial branches perform the following operations in the downstreamdirection:

-   a) the downstream bands (D2, D3, D4) belonging to the branches    D2+U2, D3+U3, and D4+U4 respectively, are up-converted back to the    original CATV spectrum's downstream bands; b) the up-converted bands    are inserted into the CATV spectrum's original frequency band. The    back-converters 210, 220, and 228 perform the following operation in    regard to the upstream bands: a) the CATV original upstream band    (the 5 to 42 MHz band) is received and up-converted to the upstream    band located in the supplementary frequency band. Consequently the    different coaxial branches will receive the following frequency    bands:    -   Branch (D1+U1) receives the CATV 50–660 MHz+D1+U1    -   Branch (D2+U2) receives the CATV 50–660 MHz+D2+U2    -   Branch (D3+U3) receives the CATV 50–660 MHz+D3+U3    -   Branch (D4+U4) receives the CATV 50–660 MHz+D4+U4

Note should be taken that the downstream frequency bands D1, D2, D3, D4could carry different information content to the subscribers served bythe respective branches.

As the demand for the downstream and the upstream depends on thepenetration of the new digital services it would be cost effective tosplit the nodes according to the increase in the current demand insteadof performing expensive node splitting for future requirements. TheSpectral Node Splitting system and method provides an optimal solutionby allowing a gradual upgrade of the network. The nodes can be splitincrementally. When the demand grows new converter/amplifier units couldbe added at the suitable locations and thereby opening up “new” coaxialbranches. Spectral Node Splitting can begin by splitting a heavilyloaded branch and could continue with other branches until finally thewhole coaxial plant is covered. Eventually the coaxial plant will beupgraded to such an extent that for every final tap of every home passeda 750 MHz bandwidth will be provided.

The performance of the Spectral Node Spitting requires novel types of RFunits such as amplifiers, frequency converters, passive elements, andthe like. The novel RF elements will have broadband capabilities andwill be able to carry multi-channel, broadband digital signals.

The converter/amplifier 198 of FIG. 7 and the back-converter 210, 220,228 of FIG. 7 are bi-directional converters/amplifiers operating betweenthe fiber optic plant and the coaxial plant. The converter 198 of FIG. 7and the back converters 210, 220, 228 of FIG. 7 are capable of handlingbroadband digital signals, offer transparent pass-through to the CATVdownstream and upstream bands, and provide very high fidelity broadbandamplification. FIG. 8 provides a functional overview of the converters.The converter includes a D2+U2 unit 258, a D3+U3 unit 260, and a D4+U4unit 262. The unit D2+U2 (258) consists of a triplexer unit 301, anupstream amplifier 312, an upstream frequency converter 301, a upstreamfilter 308, a voltage controlled oscillator 314, a downstream amplifier324, a downstream filter 320, a downstream frequency converter 318, adownstream filter 316, a power connector 290, a CATV input/output port294, a remote interface port 291, a downstream fiber port 288, anupstream fiber port 284, a coaxial plant port 296, a combined downstreaminput port 295, a downstream output port 292, a combined upstream outputport 282, and an upstream input port 280. The triplexer unit 301includes an upstream sub-unit 302, a downstream sub-unit 304, and a CATVsub-unit 306. The D3+U3 (260) unit and the D4+U4 (262) unit havesubstantially the same structure and functionalities.

The CATV signal 298 occupying a frequency band of about 5 to 860 MHz isfed into the D2+U2 unit 258 from the cable plant via a CATV amplifier300 and the CATV input/output port 294. The CATV signal is fed into thetriplexer unit 301 and passed intact by a CATV band filter (not shown)Installed in the CATV sub-unit 306 of the triplexer 301. The CATV signalis passed to the output port 296 to be transmitted combined with threedifferent downstream bands to the coaxial plant. The downstream inputsignal from the fiber plant is fed into the unit 258 via the fiber inputport 288. The downstream signal is suitably filtered by the downstreamfilter 316 to extract the appropriate downstream frequency band of about660 to 860 MHz. The extracted band is up-converted to the appropriatedownstream frequency band (D2) by the frequency converter 318 controlledby the VCO 314, filtered by the filter unit 320, and sent via thedownstream output port 292 to a splitter 293 to be combined with twodifferent downstream frequency bands sent from the units 260, and 262respectively. The combined signal of the three different downstreamfrequency bands is transmitted over the frequencies of about 1 to 2 GHzinto the unit 258 via the combined downstream input port 295. Thecombined signal is amplified by the amplifier 324, and fed to thedownstream sub-unit 304 of the triplexer 301. The signal is multiplexedwith the CATV band to form a multiplexed signal having a substantiallyextended transmission frequency bandwidth of about 5 MHz to about 3 GHz.The multiplexed signal 300 is sent to the coaxial plant through thecoaxial plant input/output port 296.

The upstream signal 298 includes the four upstream frequency bandslocated in the supplementary portion of the extended frequency rangeover the frequencies of about 2 to 2.5 GHz. The upstream signal 298 isfed from the coaxial plant via the coaxial plant input/output port 296.The upstream bands are separated from the upstream signal 298 by asuitable band-pass filter (not shown) of the upstream sub-unit 302 ofthe triplexer 301 and are fed to a splitter 261 via the combinedupstream output port 282. The upstream bands are split to threedifferent bands by the splitter 261. The split frequency bands are sentto units D2+U2 (282) via port 280, D3+U3 (260) via port 268, and D4+U4(262) via port 272 for down-conversion back into the original U2, U3, U4upstream frequency bands. The units 258, 260, and 262 send theappropriately down-converted frequency bands to the fiber plant via thefiber output port 284, 264, and 274 respectively.

The method and system proposed by the present invention uses frequencymultiplexing in an intelligent manner which improves performance andincreases the channel capacity of a specific distribution node. Athigher frequencies cable losses are higher than at lower frequencies.Therefore, the spectral node splitting method is designed to operate insuch a manner that the lowest frequencies of the frequency spectrum aresent to and are terminated in the farthest branches of the coaxial plantin contrast to higher frequencies that are sent to and are terminated inthe nearer branches.

It would be easily understood that the particular structure and theparticular functionality of the converter unit described above areexemplary only. In other embodiments of the present invention diverseother elements could be added, elements shown and described could bedropped and various advanced functions could be added. For example thesystem and method could be modified to handle more or less than theabove-described four fiber inputs, and/or the processing units D2+U2(258), D3+U3 (262), and D4+U4 (262) could be combined into a singleprocessing unit. Other useful and advanced features could be implementedwithout substantially departing from the scope and the spirit of theinvention.

Persons skilled in the art will appreciate that the present invention isnot limited to what has been particularly shown and describedhereinabove. Rather the scope of the present invention is defined onlyby the claims, which follow.

1. In a communications network accommodating at least two subscriberslinked via at least one communications network branch to at least onecontent distribution unit, the content distribution unit feeding atwo-way radio frequency signal to the at least one communicationsnetwork branch, a system of spectrally splitting the at least onedistribution unit in order to replace the at least one communicationsnetwork branch by at least two separate communications network branches,the system comprising the elements of: at least one extended converterunit to receive via an at least one optical signal conduit an at leastone two-way optical signal carrying content information having apre-determined transmission frequency bandwidth, to form a RF signalhaving a substantially extended transmission spectrum bandwidth, toconvert pre-determined downstream portions of the at least one two-wayoptical signal into at least two different pre-determined RFtransmission frequency bands, and to introduce the at least twoconverted different RF bands into a multiplexed downstream RF signalhaving a substantially extended transmission spectrum bandwidth; and atleast two extended broadband amplifiers to selectively pass at least twopre-determined RF bands from the multiplexed RF signal having asubstantially extended transmission spectrum bandwidth downstream to theat least two different communications network branches; and at least oneback converter unit to receive via the at least one communicationsnetwork branch at least one branch-specific RF signal, to extract fromthe at least one branch-specific RF at least one predetermined upstreamfrequency band, to convert the at least one branch-specific upstream RFband to a predetermined upstream frequency band, and to introduce theconverted upstream RF band into the multiplexed RF signal having asubstantially extended transmission bandwidth to be delivered upstreamto the at least one content distribution unit.
 2. The system of claim 1wherein, the at least one extended converter unit comprises the elementsof: a first converter unit to convert the first predetermined frequencyband associated with the at least one optical signal into the first RFband and to introduce the first converted RF band into the multiplexedRF signal having a substantially extended frequency bandwidth; and asecond converter unit to convert the second predetermined frequency bandassociated with the at least one optical signal into the second RF bandand to introduce the second converted RF band into the multiplexed RFsignal having a substantially extended frequency bandwidth; and a thirdconverter unit to convert the third predetermined frequency bandassociated with the at least one optical signal into the third RF bandand to introduce the third converted RF band into the multiplexed RFsignal having a substantially extended frequency bandwidth.
 3. Thesystem of claim 2, wherein the at least one extended converter unitfurther comprises the elements of: a first splitter unit to separate thefirst, the second, and the third upstream frequency bands; and a secondsplitter unit to combine the first, the second, and the thirdup-converted downstream frequency bands; and a CATV amplifier unit todrive a CATV signal into the first converter unit.
 4. The system ofclaim 2 wherein, the first converter unit comprises the elements of: atriplexer unit to multiplex the up-converted downstream frequency bandsand the CATV signal into a multiplexed extended signal and tode-multiplex the upstream frequency bands from the multiplexed extendedsignal; and an upstream amplifier unit to drive the first upstreamfrequency band; and an upstream filter unit for filtering the firstupstream frequency band; and an upstream frequency converter unit todown-convert the first upstream frequency band; and a downstreamfrequency converter unit to up-convert the first downstream frequencyband; and a downstream filter unit for filtering the first downstreamfrequency band; and a downstream amplifier unit to drive the combineddownstream frequency bands; and a voltage controlled oscillator tocontrol the upstream frequency controller and the downstream frequencycontroller.
 5. The system of claim 2 wherein, the second converter unitcomprises elements substantially identical to the elements constitutingthe first converter unit.
 6. The system of claim 5 wherein theoperational parameters associated with the elements of the secondconverter unit are pre-determined for the appropriate handling of thesecond downstream band and the second upstream band.
 7. The system ofclaim 2 wherein the third converter unit comprises elementssubstantially identical to the elements constituting the first converterunit.
 8. The system of claim 7 wherein the operational parametersassociated with the elements of the third converter unit arepre-determined for the appropriate handling of the third downstream bandand the third upstream band.
 9. The system of claim 1, wherein the atleast one extended back converter unit comprises the elements of: afirst back converter unit to extract the first upstream frequency bandfrom the first branch-specific frequency spectrum, to convert the firstupstream frequency band and to introduce the first converted upstreamfrequency band into the multiplexed extended frequency band to bedelivered to the at least one distribution unit; and a second backconverter unit to extract the second upstream frequency band from thesecond branch-specific frequency spectrum, to convert the secondupstream frequency band and to introduce the second converted upstreamfrequency band into the multiplexed extended frequency band to bedelivered to the at least one distribution unit; and a third backconverter unit to extract the third upstream frequency band from thethird branch-specific frequency spectrum, to convert the third upstreamfrequency band and to introduce the third converted upstream frequencyband into the multiplexed extended frequency band to be delivered to theat least one distribution unit.
 10. The system of claim 9 wherein, thefirst back-converter unit comprises elements substantially identical tothe elements constituting the extended converter unit.
 11. The system ofclaim 10 wherein, the second back-converter unit comprises elementssubstantially identical to the elements constituting the firstback-converter converter unit.
 12. The system of claim 11 wherein, thethird back-converter unit comprises elements substantially identical tothe elements constituting the first back-converter converter unit. 13.The system of claim 11 wherein the operational parameters associatedwith the elements of the second back-converter unit are pre-determinedfor the appropriate handling of the second downstream band and thesecond upstream band.
 14. The system of claim 12 wherein the operationalparameters associated with the elements of the third back-converter unitare pre-determined for the appropriate handling of the third downstreamband and the third upstream band.
 15. The system of claim 1 wherein thecommunications network is a cable TV system based on a hybridfiber-optic infrastructure.
 16. The system of claim 1 wherein thecommunications network is a satellite communications network.
 17. Thesystem of claim 1 wherein the communications network is a Local AreaNetwork network.
 18. The system of claim 1 wherein the communicationsnetwork is a Wide Area Network.
 19. The system of claim 1 wherein theextended converter unit comprises one or more converter units.
 20. In acommunication network accommodating at least two subscribers linked viaat least one communications network branch to at least one contentdistribution unit, the content distribution unit feeding a two-way RFsignal to the at least one communications network branch, a method ofspectrally splitting the at least one content distribution unit in orderto replace the at least one communications network branch by at leasttwo separate communications network branches, the method comprising thesteps of: receiving at least two optical signals carrying encodedcontent information in at least two predefined different downstreamfrequency bands; and converting the at least two pre-defined downstreamfrequency band to at least two pre-determined converted frequency bandwithin the combined broadband signal; and converting the at least twopre-defined upstream frequency band to the at least two pre-determinedfrequency band within the combined broadband signal; and multiplexingthe CATV signal and the at least two converted downstream frequency bandinto a combined broadband signal having a substantially extendedfrequency range; and selectively distributing the converted downstreamfrequency bands within the combined broadband signal to separatecommunications network branches; and selectively receiving at least twoupstream frequency bands included in the combined broadband signal andtransmitted from separate communications network branches; andconverting the at least two upstream frequency bands to pre-definedfrequency bands.
 21. The method of claim 20 further comprises the stepsof: determining the size and the frequency ranges of the converteddownstream frequency bands in the combined broadband signal; anddetermining the size and the frequency ranges of the converted upstreamfrequency ranges in the combined broadband signal.
 22. The method ofclaim 20 further comprises the steps of: receiving an optical signalcontaining an encoded CATV signal in a pre-defined frequency band; anddistributing the CATV portion of the combined broadband signal intoseparate communications network branches; and transmitting the convertedupstream frequency bands to the network plant.
 23. The method of claim20 wherein the combined broadband signal is having an extended frequencyrange of about 5 MHz to 3 GHz.
 24. The method of claim 20, wherein theconverted downstream frequency ranges are multiplexed into the combinedbroadband signal over a frequency range of about 1 GHz to 2 GHz.
 25. Themethod of claim 20 wherein the converted upstream frequency ranges aremultiplexed into the combined broadband signal over a frequency range ofabout 2 GHz to 3 GHz.
 26. The method of claim 20 wherein the combinedbroadband signal is formed by attaching to the CATV transmissionspectrum of about 5 to 860 MHz a supplementary frequency band of about 1to 2 GHz to hold the converted downstream frequency ranges and theconverted upstream frequency ranges.
 27. The method of claim 20 whereinthe downstream frequency bands are allocated a frequency bandwidth ofabout 160 MHz to 200 MHz.
 28. The method of claim 20 wherein theupstream frequency bands are allocated a frequency bandwidth of about 30MHz.